Polymerization Process

ABSTRACT

The present invention relates to a process for continuously preparing water-absorbing polymers by mixing a monomer solution with at least one crosslinker and polymerizing the resulting mixture, wherein the residence time of the mixture between the addition of the at least one crosslinker and the entry into the polymerization reactor is less than 180 seconds, and also to an apparatus for performing the process.

The present invention relates to a process for continuously preparingwater-absorbing polymers by mixing a monomer solution with at least onecrosslinker and polymerizing the resulting mixture, wherein theresidence time of the mixture between the addition of the at least onecrosslinker and the entry into the polymerization reactor is less than180 seconds, and also to an apparatus for performing the process.

Further embodiments of the present invention can be taken from theclaims, the description and the examples. It is evident that thefeatures of the inventive subject matter which have been mentioned aboveand will be explained below are usable not only in the combinationspecified in each case but also in other combinations without leavingthe scope of the invention.

Water-absorbing polymers are especially polymers of (co)polymerizedhydrophilic monomers, graft (co)polymers of one or more hydrophilicmonomers on a suitable graft base, crosslinked cellulose ethers orstarch ethers, crosslinked carboxymethylcellulose, partly crosslinkedpolyalkylene oxide or natural products swellable in aqueous liquids, forexample guar derivatives, preference being given to water-absorbingpolymers based on partly neutralized acrylic acid. Such polymers areused as products that absorb aqueous solutions to produce diapers,tampons, sanitary napkins and other hygiene articles, but also aswater-retaining agents in market gardening.

The preparation of the water-absorbing polymers is described, forexample, in the monograph “Modern Superabsorbent Polymer Technology”, F.L. Buchholz and A. T. Graham, Wiley-VCH, 1998, or in Ullmann'sEncyclopedia of Industrial Chemistry, 6th Edition, Volume 35, pages 73to 103. The preferred preparation process is solution or gelpolymerization. In this technology, a monomer mixture is firstlyprepared and is neutralized batchwise and then transferred to apolymerization reactor, or initially charged actually within thepolymerization reactor. In the batchwise or continuous process whichfollows, the reaction is effected to give the polymer gel which, in thecase of a stirred polymerization, is already in comminuted form. Thepolymer gel is subsequently dried, ground and sieved and thentransferred to further surface treatment.

A continuous polymerization process forms the basis, for example, ofWO-A-01/38402, in which the aqueous monomer solution together with theinitiator and the inert gas is fed continuously to a mixing kneader withat least two axially parallel-rotating shafts.

Continuous gel polymerizations are also known from WO-A-03/004237,WO-A-03/022896 and WO-A-01/016197.

It was an object of the present invention to provide an improvedpolymerization process for preparing crosslinked polymers, in which thecrosslinker conversion should be at a maximum and the proportion ofuncrosslinked polymers should be at a minimum.

The object is achieved by a process for continuously preparingwater-absorbing polymers by mixing a monomer solution with at least onecrosslinker and polymerizing the resulting mixture, wherein theresidence time of the mixture between the addition of the at least onecrosslinker and the entry into the polymerization reactor is less than180 seconds.

The residence time of the mixture between the addition of the at leastone crosslinker and the entry into the polymerization reactor ispreferably less than 120 seconds, preferentially less than 60 seconds,more preferably less than 30 seconds, most preferably less than 10seconds. A very particularly advantageous residence time is in the rangefrom 1 to 5 seconds.

The polymerization tendency can be reduced when the connection betweencrosslinker metering and polymerization reactor at least partly,preferably at least to an extent of at least 50% of the surface, morepreferably as completely as possible in construction terms, has amaterial surface which has a contact angle for water of at least 60°,preferably at least 90°, more preferably at least 100°.

The contact angle is a measure of the wetting behavior and can bemeasured by customary methods, preferably according to DIN 53900.

Suitable materials with corresponding wetting behavior are polyethylene,polypropylene, polyester, polyamide, polytetrafluoroethylene, polyvinylchloride, epoxy resins and silicone resins. Very particular preferenceis given to polypropylene.

The process according to the invention is particularly advantageous whenthe crosslinker, depending on the type and amount, is not completelysoluble in the monomer solution and is present at least partly indispersed form in the monomer solution. The presence of a dispersion canbe determined readily by scattered light measurements.

The viscosity of the monomer solution at 15° C. is preferably from 5 to200 mPas, more preferably from 10 to 100 mPas, most preferably from 20to 50 mPas, the viscosity being measured with a Brookfield viscometer(spindle 2, 100 rpm).

The monomer concentration in the monomer solution is preferably from 10to 80% by weight, more preferably from 20 to 60% by weight, mostpreferably from 30 to 50% by weight.

The monomer solution comprises at least one monoethylenicallyunsaturated monomer, preferably acrylic acid and/or salts thereof. Theproportion of acrylic acid and/or salts thereof in the total amount ofmonomer is preferably at least 50 mol %, more preferably at least 90 mol%, most preferably at least 95 mol %.

In a preferred embodiment of the present invention, the at least onecrosslinker is metered in via a Venturi tube.

A Venturi tube is a tube constriction of restricted length, in which thepressure drop is converted substantially reversibly to kinetic energy.To this end, the cross-sectional area F₁ is reduced over the distance L₁(narrowing zone) to the cross section F₂, the cross-sectional area F₂ iskept constant over the distance L₂ (constriction zone) and thecross-sectional area F₂ is then widened again over the distance L₃(diffuser) to the cross-sectional area F₁. The cross-sectional area F₁is greater than the cross-sectional area F₂ and the length L₃ greaterthan the length L₁.

The crosslinker is metered in preferably in the region of the zone L₁ orof the zone L₂.

FIG. 1 shows a typical Venturi tube, the reference symbols having thefollowing meaning:

A: monomer solution before metering of crosslinker

B: crosslinker feed

C: monomer solution comprising crosslinker

L₁: narrowing zone

L₂: constriction zone

L₃: diffuser

D₁: diameter of the pipeline

D₂: diameter of the constriction zone

The optimal design of a Venturi tube is known per se to those skilled inthe art. The Venturi tube is preferably designed such that the pressurein the region of the zone L₂ is less than the ambient pressure (suctionconveying) and/or that the flow in the region of the zone L₂ isturbulent, and the Reynolds number should be at least 1000, preferablyat least 2000, more preferably at least 3000, most preferably at least4000, and typically less than 10 000 000.

The at least one crosslinker can be metered in via one or more additionpoints. For example, the reactants can be metered in via two, three,four, five or six addition points, in which case the addition points arepreferably arranged such that they have a common axis (for two additionpoints) or form a symmetrical star (for at least three addition points)and the axis or star is at right angles to the flow direction of themonomer solution (multiple addition points).

The division into a plurality of addition points brings about moreuniform mixing.

When a plurality of crosslinkers is used, they may be metered inseparately or as a mixture.

It is also possible to mix the at least one crosslinker first with aportion of the monomer solution and then to mix this mixture with themajority of the monomer solution.

Preference is given to mixing a preneutralized monomer solution with theat least one crosslinker, inertizing the mixture, mixing the inertizedmixture with an initiator and polymerizing.

The water-absorbing polymers are obtained, for example, bypolymerization of a monomer solution comprising

-   -   a) at least one ethylenically unsaturated acid-functional        monomer,    -   b) at least one crosslinker,    -   c) if appropriate one or more ethylenically and/or allylically        unsaturated monomers copolymerizable with the monomer a), and    -   d) if appropriate one or more water-soluble polymers onto which        the monomers a), b) and if appropriate c) can be at least partly        grafted.

Suitable monomers a) are, for example, ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid, maleic acid,fumaric acid and itaconic acid, or derivatives thereof, such asacrylamide, methacrylamide, acrylic esters and methacrylic esters.Particularly preferred monomers are acrylic acid and methacrylic acid.Very particular preference is given to acrylic acid.

The monomers a), especially acrylic acid, comprise preferably up to0.025% by weight of a hydroquinone monoether. Preferred hydroquinonemonoethers are hydroquinone monomethyl ether (MEHQ) and/or tocopherols.

Tocopherol refers to compounds of the following formula:

where R¹ is hydrogen or methyl, R² is hydrogen or methyl, R³ is hydrogenor methyl and R⁴ is hydrogen or an acyl radical having from 1 to 20carbon atoms.

Preferred R⁴ radicals are acetyl, ascorbyl, succinyl, nicotinyl andother physiologically tolerable carboxylic acids. The carboxylic acidsmay be mono-, di- or tricarboxylic acids.

Preference is given to alpha-tocopherol where R¹═R²═R³=methyl,especially racemic alpha-tocopherol. R¹ is more preferably hydrogen oracetyl. Especially preferred is RRR-alpha-tocopherol.

The monomer solution comprises preferably not more than 130 ppm byweight, more preferably not more than 70 ppm by weight, preferably notless than 10 ppm by weight, more preferably not less than 30 ppm byweight and especially about 50 ppm by weight of hydroquinone monoether,based in each case on acrylic acid, with acrylic acid salts beingcounted as acrylic acid. For example, the monomer solution can beprepared using acrylic acid having an appropriate hydroquinone monoethercontent.

The crosslinkers b) are compounds having at least two polymerizablegroups which can be free-radically polymerized into the polymer network.Suitable crosslinkers b) are, for example, ethylene glycoldimethacrylate, diethylene glycol diacrylate, allyl methacrylate,trimethylolpropane triacrylate, triallylamine, tetraallyloxyethane, asdescribed in EP-A-0 530 438, di- and triacrylates, as described inEP-A-0 547 847, EP-A-0 559 476, EP-A-0 632 068, WO-A-93/21237,WO-A-03/104299, WO-A-03/104300, WO-A-03/104301 and DE-A-103 31 450,mixed acrylates which, as well as acrylate groups, comprise furtherethylenically unsaturated groups, as described in DE-A-103 31 456 andWO-A-04/013064, or crosslinker mixtures as described, for example, inDE-A-195 43 368, DE-A-196 46 484, WO-A-90/15830 and WO-A-02/32962.

Suitable crosslinkers b) include in particularN,N′-methylenebisacrylamide and N,N′-methylenebismethacrylamide, estersof unsaturated mono- or polycarboxylic acids of polyols, such asdiacrylate or triacrylate, for example butanediol diacrylate, butanedioldimethacrylate, ethylene glycol diacrylate, ethylene glycoldimethacrylate and also trimethylolpropane triacrylate and allylcompounds, such as allyl(meth)acrylate, triallyl cyanurate, diallylmaleate, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallylethylenediamine, allyl esters of phosphoric acid and alsovinylphosphonic acid derivatives as described, for example, in EP-A-0343 427. Suitable crosslinkers b) further include pentaerythritoldiallyl ether, pentaerythritol triallyl ether, pentaerythritoltetraallyl ether, polyethylene glycol diallyl ether, ethylene glycoldiallyl ether, glycerol diallyl ether, glycerol triallyl ether,polyallyl ethers based on sorbitol, and also ethoxylated variantsthereof. In the process of the invention, it is possible to usedi(meth)acrylates of polyethylene glycols, the polyethylene glycol usedhaving a molecular weight between 300 and 1000.

However, particularly advantageous crosslinkers b) are di- andtriacrylates of 3- to 15-tuply ethoxylated glycerol, of 3- to 15-tuplyethoxylated trimethylolpropane, of 3- to 15-tuply ethoxylatedtrimethylolethane, especially di- and triacrylates of 2- to 6-tuplyethoxylated glycerol or of 2- to 6-tuply ethoxylated trimethylolpropane,of 3-tuply propoxylated glycerol or of 3-tuply propoxylatedtrimethylolpropane, and also of 3-tuply mixed ethoxylated orpropoxylated glycerol or of 3-tuply mixed ethoxylated or propoxylatedtrimethylolpropane, of 15-tuply ethoxylated glycerol or of 15-tuplyethoxylated trimethylolpropane, of 40-tuply ethoxylated glycerol, of40-tuply ethoxylated trimethylolethane or of 40-tuply ethoxylatedtrimethylolpropane.

Very particularly preferred crosslinkers b) are polyethoxylated and/or-propoxylated glycerols which have been esterified with acrylic acid ormethacrylic acid to di- or triacrylates, as described, for example, inWO 03/104301. Di- and/or triacrylates of 3- to 10-tuply ethoxylatedglycerol are particularly advantageous. Very particular preference isgiven to di- or triacrylates of 1- to 5-tuply ethoxylated and/orpropoxylated glycerol. The triacrylates of 3- to 5-tuply ethoxylatedand/or propoxylated glycerol are most preferred. These are notable forparticularly low residual levels (typically below 10 ppm by weight) inthe water-absorbing polymer and the aqueous extracts of thewater-absorbing polymers produced therewith have an almost unchangedsurface tension (typically not less than 0.068 N/m) compared with waterat the same temperature.

The amount of crosslinker b) is preferably from 0.01 to 1% by weight,more preferably from 0.05 to 0.5% by weight, most preferably from 0.1 to0.3% by weight, obtained in each case on the monomer a).

Examples of ethylenically unsaturated monomers c) which arecopolymerizable with the monomers a) are acrylamide, methacrylamide,crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate anddimethylaminoneopentyl methacrylate.

Useful water-soluble polymers d) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols orpolyacrylic acids, preferably polyvinyl alcohol and starch.

For optimal action, the preferred polymerization inhibitors requiredissolved oxygen. Typically, the monomer solutions are substantiallyfreed of oxygen before the polymerization (inertization), for example bymeans of flowing an inert gas, preferably nitrogen, through them. Thisdistinctly weakens the action of the polymerization inhibitors. Theoxygen content of the monomer solution is preferably lowered to lessthan 1 ppm by weight and more preferably to less than 0.5 ppm by weightbefore the polymerization.

The preparation of a suitable base polymer and also further suitablehydrophilic ethylenically unsaturated monomers d) are described inDE-A-199 41 423, EP-A-0 686 650, WO-A-01/45758 and WO-A-03/104300.

Water-absorbing polymers are typically obtained by additionpolymerization of an aqueous monomer solution and, if appropriate,subsequent comminution of the hydrogel. Suitable preparation methods aredescribed in the literature. Water-absorbing polymers are obtainable,for example, by

-   -   gel polymerization in the batch process or tubular reactor and        subsequent comminution in meat grinder, extruder or kneader        (EP-A-0 445 619, DE-A-19 846 413)    -   addition polymerization in kneader with continuous comminution        by contrarotatory stirring shafts for example (WO-A-01/38402)    -   addition polymerization on belt and subsequent comminution in        meat grinder, extruder or kneader (DE-A-38 25 366, U.S. Pat. No.        6,241,928)    -   emulsion polymerization, which produces bead polymers having a        relatively narrow gel size distribution (EP-A-0 457 660)    -   in situ addition polymerization of a woven fabric layer which,        usually in a continuous operation, has previously been sprayed        with aqueous monomer solution and subsequently been subjected to        a photopolymerization (WO-A-02/94328, WO-A-02/94329).

The reaction is preferably carried out in a kneader, as described, forexample, in WO-A-01/38402, or on a belt reactor, as described, forexample, in EP-A-0 955 086.

Neutralization can also be carried out partly after the polymerization,at the hydrogel stage. It is therefore possible to neutralize up to 40mol %, preferably from 10 to 30 mol % and more preferably from 15 to 25mol % of the acid groups before the polymerization by adding a portionof the neutralizing agent to the monomer solution and setting thedesired final degree of neutralization only after the polymerization, atthe hydrogel stage. The monomer solution can be neutralized by mixing inthe neutralizing agent. The hydrogel may be comminuted mechanically, forexample by means of a meat grinder, in which case the neutralizing agentcan be sprayed, sprinkled or poured on and then carefully mixed in. Tothis end, the gel mass obtained can be repeatedly ground in the meatgrinder for homogenization. Neutralization of the monomer solution tothe final degree of neutralization is preferred.

The neutralized hydrogel is then dried with a belt or drum dryer untilthe residual moisture content is preferably below 15% by weight andespecially below 10% by weight, the water content being determined byEDANA (European Disposables and Nonwovens Association) recommended testmethod No. 430.2-02 “Moisture content”. If desired, drying can also becarried out using a fluidized bed dryer or a heated plowshare mixer. Toobtain particularly white products, it is advantageous to dry this gelwhile ensuring rapid removal of the evaporating water. To this end, thedryer temperature must be optimized, the air feed and removal has to becontrolled, and sufficient venting must be ensured in each case. Thehigher the solids content of the gel, the simpler the drying, by itsnature, and the whiter the product. The solids content of the gel beforethe drying is therefore preferably between 30% and 80% by weight. It isparticularly advantageous to vent the dryer with nitrogen or anothernonoxidizing inert gas. If desired, however, it is possible simply justto lower the partial pressure of the oxygen during the drying in orderto prevent oxidative yellowing processes. In general, though, adequateventing and removal of the water vapor also still lead to an acceptableproduct. A very short drying time is generally advantageous with regardto color and product quality.

The dried hydrogel is preferably ground and sieved, useful grindingapparatus typically including roll mills, pin mills or swing mills. Theparticle size of the sieved, dry hydrogel is preferably below 1000 μm,more preferably below 900 μm and most preferably below 800 μm, andpreferably above 100 μm, more preferably above 150 μm and mostpreferably above 200 μm.

Very particular preference is given to a particle size (sieve cut) offrom 106 to 850 μm. The particle size is determined according to EDANA(European Disposables and Nonwovens Association) recommended test methodNo. 420.2-02 “Particle size distribution”.

The base polymers are then preferably surface postcrosslinked.Postcrosslinkers suitable for this purpose are compounds comprising twoor more groups capable of forming covalent bonds with the carboxylategroups of the hydrogel. Suitable compounds are, for example, alkoxysilylcompounds, polyaziridines, polyamines, polyamidoamines, di- orpolyglycidyl compounds, as described in EP-A-0 083 022, EP-A-543 303 andEP-A-937 736, di- or polyfunctional alcohols, as described in DE-C-33 14019, DE-C-35 23 617 and EP-A-450 922, or β-hydroxyalkylamides, asdescribed in DE-A-102 04 938 and U.S. Pat. No. 6,239,230.

In addition, DE-A-40 20 780 describes cyclic carbonates, DE-A-198 07 5022-oxazolidone and its derivatives, such as 2-hydroxyethyl-2-oxazolidone,DE-A-198 07 992 bis- and poly-2-oxazolidinones, DE-A-198 54 5732-oxotetrahydro-1,3-oxazine and its derivatives, DE-A-198 54 574N-acyl-2-oxazolidones, DE-A-102 04 937 cyclic ureas, DE-A-103 34 584bicyclic amide acetals, EP-A-1 199 327 oxetanes and cyclic ureas andWO-A-03/031482 morpholine-2,3-dione and its derivatives, as suitablesurface postcrosslinkers.

The postcrosslinking is typically carried out in such a way that asolution of the surface postcrosslinker is sprayed onto the hydrogel oronto the dry base polymer powder. After the spraying, the polymer powderis dried thermally, and the crosslinking reaction may take place eitherbefore or during drying.

The spraying with a solution of the crosslinker is preferably carriedout in mixers having moving mixing implements, such as screw mixers,paddle mixers, disk mixers, plowshare mixers and shovel mixers.Particular preference is given to vertical mixers and very particularpreference to plowshare mixers and shovel mixers. Suitable mixers are,for example, Lödige® mixers, Bepex® mixers, Nauta® mixers, Processall®mixers and Schugi® mixers.

The thermal drying is preferably carried out in contact dryers, morepreferably shovel dryers and most preferably disk dryers. Suitabledryers are, for example, Bepex® dryers and Nara® dryers. It is alsopossible to use fluidized bed dryers.

The drying can be effected in the mixer itself, by heating the jacket orblowing in warm air. It is equally possible to use a downstream dryer,for example a tray dryer, a rotary tube oven or a heatable screw. It isalso possible, for example, to utilize an azeotropic distillation as adrying process.

Preferred drying temperatures are in the range from 50 to 250° C.,preferably in the range from 50 to 200° C. and more preferably in therange from 50 to 150° C. The preferred residence time at thistemperature in the reaction mixer or dryer is below 30 minutes and morepreferably below 10 minutes.

The process according to the invention enables the economically viablecontinuous preparation of postcrosslinked water-absorbing polymerparticles. The crosslinkers used are utilized efficiently. Theproportion of unconverted crosslinkers and of un-crosslinked polymers isparticularly low.

The present invention further provides an apparatus for the performanceof the process according to the invention, comprising

i) a polymerization reactor,

ii) at least one inlet to the polymerization reactor i),

iii) at least one Venturi tube in the inlet ii) and

iv) at least one inlet to the Venturi tube iii),

the inlet iv) opening into the Venturi tube iii), preferably into thenarrowing zone.

Advantageously, the inner surface of the inlet ii) betweenpolymerization reactor i) and inlet iii) at least partly has a contactangle for water of at least 60°, preferably at least 90°, morepreferably at least 100°.

The contact angle is a measure of the wetting behavior and can bemeasured by customary methods, preferably according to DIN 53900.

Suitable materials with corresponding wetting behavior are polyethylene,polypropylene, polyester, polyamide, polytetrafluoroethylene, polyvinylchloride, epoxy resins and silicone resins. Very particular preferenceis given to polypropylene.

The length of the inlet ii) between polymerization reactor i) and inletiii) is preferably from 0.5 to 20 m, more preferably from 1 to 10 m,most preferably from 1.5 to 5 m.

The cross-sectional area of the inlet ii) is preferably from 10 to 1000cm², more preferably from 25 to 500 cm², most preferably from 50 to 200cm². The inlet ii) preferably has a circular cross section.

Preferably at least two inlets iii) are present, more preferably two,three, four, five or six inlets iii), the inlets iii) preferably beingarranged such that they have a common axis (for two inlets) or form asymmetrical star (for at least three inlets), and the axis or star is atright angles to the flow direction of the monomer solution (multipleaddition points).

Particularly advantageously, two, three or four multiple addition pointsare arranged in succession.

For example, at least eight inlets iii) may be present, in which casefour inlets iii) open in a cross shape into the monomer line; the atleast 2 groups of four inlets iii) being arranged in succession andoffset relative to one another.

In a preferred embodiment, the inlets iii) are arranged such that theangle between the inlet ii) and the inlet iii) in flow direction is lessthan 90°. The angle is preferably from 10 to 80°, more preferably from20 to 70°, most preferably from 30 to 60°.

The apparatus is preferably free of dead spaces and the surfaces shouldhave minimum roughness.

Dead spaces are sections of the apparatus in which the average residencetime is increased in the course of operation as intended.

The inventive apparatus is outstandingly suitable for meteringcrosslinkers into monomer solutions. Especially owing to their specificinner surface, the polymerization tendency is low.

Methods:

The measurements should, unless stated otherwise, be carried out at anambient temperature of 23±2° C. and a relative humidity of 50±10%. Thewater-absorbing polymers are mixed thoroughly before the measurement.

Residual Crosslinkers

The content of residual crosslinkers of the water-absorbing polymerparticles is determined by means of HPLC using a ZORBAX® Eclipse XDB C18reverse-phase column (Agilent Technologies, US) with downstream UVNISdetection and calibration with external standard. The mobile phase usedis acetonitrile/water with a gradient.

Extractables

The content of extractables in the water-absorbing polymer particles isdetermined in accordance with the EDANA (European Disposables andNonwovens Association) recommended test method No. 470.2-02“Extractables”.

The EDANA test methods are obtainable, for example, from the EuropeanDisposables and Nonwovens Association, Avenue Eugene Plasky 157, B-1030Brussels, Belgium.

EXAMPLES Example 1

Continuous mixing of water, 50% by weight sodium hydroxide solution andacrylic acid prepared a 38.8% by weight acrylic acid/sodium acrylatesolution with a degree of neutralization of 71.3 mol %. As thecomponents were mixed, the monomer solution was cooled to a temperatureof 29° C. continuously by a heat exchanger.

The polyethylenically unsaturated crosslinker used was polyethyleneglycol diacrylate (diacrylate of a polyethylene glycol having a meanmolar mass of 400 g/mol). The amount used was 2 kg per t of monomersolution. The crosslinker was metered in via one addition point. Theaddition was effected via a pipeline having a diameter of 0.5 cm. Theaddition point of the crosslinker was 1 m upstream of the reactorentrance. The residence time of the crosslinker in the monomer solutionupstream of the polymerization reactor was 1.5 seconds.

After the crosslinker, hydrogen peroxide and sodium peroxodisulfate weremetered into the monomer solution. The amounts used per t of monomersolution were 1.0 kg of 0.25% by weight hydrogen peroxide and 3.1 kg of15% by weight aqueous sodium peroxodisulfate.

The throughput of the monomer solution was 18 t/h.

The monomer mixture and ascorbic acid were metered continuously into aList Contiknet reactor (from List, Arisdorf, Switzerland). The pressurein the reactor was increased by 10 mbar relative to the environment. Theamount of 1% by weight aqueous ascorbic acid used was 1.1 kg per t ofmonomer solution.

Before being fed in, the reaction solution was degassed with nitrogenand had a temperature of 23.5° C. at the feed. The reactor was operatedwith a rotational speed of the shafts of 38 rpm. The residence time ofthe reaction mixture in the reactor was 15 minutes.

After polymerization had ended and gel comminution, the polymer gel wasplaced onto a belt dryer. During the drying, a pressure reduced by 5mbar relative to ambient pressure was set. The precomminuted polymer gelwas placed onto the belt dryer with a layer thickness of 10 cm and driedwith warm air (175° C.). The residence time in the belt dryer was 37minutes.

The resulting polymer powder was ground, sieved (from 100 to 800 μm) andsurface postcrosslinked.

The postcrosslinker used was a 1.2% by weight solution of ethyleneglycol digycidyl ether in propylene glycol/water (1:2). Based on thepolymer powder, 5% by weight of postcrosslinker solution were sprayed onand the polymer powder was aftertreated thermally at 150° C. for 60minutes.

The postcrosslinked polymer powder was analyzed. The results arecompiled in Table 1.

Example 2

The procedure of Example 1 was repeated. The addition point of thecrosslinker was 3.5 m upstream of the reactor entrance. The residencetime of the crosslinker in the monomer solution upstream of thepolymerization reactor was 5.3 seconds.

The postcrosslinked polymer powder was analyzed. The results arecompiled in Table 1.

Example 3

The procedure of Example 1 was repeated. The addition point of thecrosslinker was 2.5 m upstream of the reactor entrance. The residencetime of the crosslinker in the monomer solution upstream of thepolymerization reactor was 3.8 seconds.

The postcrosslinked polymer powder was analyzed. The results arecompiled in Table 1.

TABLE 1 Residence time of the crosslinker Residence Residual Exampletime Extractables crosslinker 1 1.5 s 10.1% by wt.  0.0110% by wt. 2 5.3s 9.2% by wt. 0.0070% by wt. 3 3.8 s 8.4% by wt. 0.0025% by wt.

The results show that residual crosslinker and extractables pass througha minimum with increasing residence time.

Example 4

The procedure of Example 1 was repeated. The addition point of thecrosslinker was 2.5 m upstream of the reactor entrance. The residencetime of the crosslinker in the monomer solution upstream of thepolymerization reactor was 3.8 seconds.

To meter in the crosslinker, a 93.2 cm-long Venturi tube was used (FIG.1), in which the pipeline narrowed from a diameter of 9 cm to 3.6 cmover a distance of 8.4 cm (zone L₁), retained the diameter of 3.6 cmover a distance of 27.6 cm (zone L₂) and widened again from a diameterof 3.6 cm to 9 cm over a distance of 57 cm (zone L₃).

The crosslinker was metered into the Venturi tube through a pipelinehaving an internal diameter of 5 mm. The pipeline ended 5 cm beyond thestart of the constriction zone.

The postcrosslinked polymer powder was analyzed. The results arecompiled in Table 2.

Example 5

The procedure of Example 4 was repeated. The crosslinker was meteredinto the Venturi tube via four pipelines having an internal diameter of5 mm. The pipelines opened 5 and 13 cm respectively beyond the start ofthe constriction zone. The pipelines were opposite one another in pairs.The pipe axes of the two pipeline pairs were rotated by 90° with respectto one another.

The postcrosslinked polymer powder was analyzed. The results arecompiled in Table 2.

Example 6

The procedure of Example 4 was repeated. The crosslinker was meteredinto the Venturi tube through eight pipelines having an internaldiameter of 5 mm. The pipelines opened 5 and 13 cm respectively beyondthe start of the constriction zone, with four pipelines in each case atright angles to one another.

TABLE 2 Number of feeds Residual Example Number of feeds Extractablescrosslinker 4 1 8.0% by wt. 0.0015% by wt. 5 4 8.3% by wt. 0.0009% bywt. 6 8 8.4% by wt. <0.0008% by wt.  

The results show that the residual crosslinker decreases with increasingnumber of feeds.

1. A process for continuously preparing water-absorbing polymers bymixing a monomer solution with at least one crosslinker and polymerizinga resulting mixture, wherein a residence time of the mixture between anaddition of the at least one crosslinker and entry into a polymerizationreactor is less than 180 seconds.
 2. The process according to claim 1,wherein the residence time of the mixture between the addition of the atleast one crosslinker and the entry into the polymerization reactor isat least one second.
 3. The process according to claim 1, wherein theinner surface of a connection between a feed of the at least onecrosslinker and the polymerization reactor at least partly has a contactangle for water of at least 60°.
 4. The process according to claim 1,wherein the at least one crosslinker is not completely soluble in themixture.
 5. The process according to claim 1, wherein the mixture isinertized.
 6. The process according to claim 1, wherein the at least onecrosslinker is metered into a monomer solution via a Venturi tube. 7.The process according to claim 1, wherein the mixture, between theaddition of the at least one crosslinker and the polymerization, flowsat least partly with a velocity which corresponds to a Reynolds numberof from 1000 to 10,000.
 8. The process according to claim 1, wherein atleast 50 mol % of the monomers of the monomer solution are acrylic acidand/or salts thereof.
 9. The process according to claim 1, wherein themonomer solution is polymerized in the polymerization reactor to give ahydrogel, dried, grounds, and classified.
 10. The process according toclaim 9, wherein the classified polymer particles are surfacepostcrosslinked.
 11. An apparatus for continuous polymerization,comprising i) a polymerization reactor, ii) at least one inlet to thepolymerization reactor i), iii) at least one Venturi tube in the inletii): and iv) at least one inlet to the Venturi tube iii), the inlet iv)opening into the Venturi tube iii) and a length of the inlet ii) betweenpolymerization reactor i) and the Venturi tube iii) being at least 0.5m.
 12. The apparatus according to claim 11, wherein the an inner surfaceof the inlet ii) between the polymerization reactor i) and the inletiii) at least partly has a contact angle for water of at least 60°. 13.The apparatus according to claim 11, wherein the length of the inlet ii)between the polymerization reactor i) and the Venturi tube iii) is from0.5 to 20 m.
 14. The apparatus according to claim 11, wherein an anglebetween the inlet ii) and the inlet iv) in a flow direction is less than90°.
 15. (canceled)
 16. The apparatus according to claim 12, wherein thelength of the inlet ii) between the polymerization reactor i) and theVenturi tube iii) is from 0.5 to 20 m.
 17. The apparatus according toclaim 12, wherein an angle between the inlet ii) and the inlet iv) in aflow direction is less than 90°.
 18. The apparatus according to claim13, wherein an angle between the inlet ii) and the inlet iv) in a flowdirection is less than 90°.
 19. The process according to claim 3,wherein the at least one crosslinker is metered into the monomersolution via a Venturi tube.
 20. The process according to claim 19,wherein the mixture, between the addition of the at least onecrosslinker and the polymerization, flows at least partly with avelocity which corresponds to a Reynolds number of from 1000 to 10,000.